The present invention relates to processing orthogonal frequency division multiplexed (OFDM) signals.
A wireless LAN (WLAN) is a flexible data communications system implemented as an extension to, or as an alternative for, a wired LAN within a building or campus. Using electromagnetic waves, WLANs transmit and receive data over the air, minimizing the need for wired connections. Thus, WLANs combine data connectivity with user mobility, and, through simplified configuration, enable movable LANs. Some industries that have benefited from the productivity gains of using portable terminals (e.g., notebook computers) to transmit and receive real-time information are the digital home networking, health care, retail, manufacturing, and warehousing industries.
Manufacturers of WLANs have a range of transmission technologies to choose from when designing a WLAN. Some exemplary technologies are multicarrier systems, spread spectrum systems, narrowband systems, and infrared systems. Although each system has its own benefits and detriments, one particular type of multicarrier transmission system, orthogonal frequency division multiplexing (OFDM), has proven to be exceptionally useful for WLAN communications.
OFDM is a robust technique for efficiently transmitting data over a channel. The technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit data. These sub-carriers are arranged for optimal bandwidth efficiency compared to conventional frequency division multiplexing (FDM) which can waste portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier.
The transmission of data through a channel via OFDM signals also provides several other advantages over more conventional transmission techniques. Some of these advantages are a tolerance to multipath delay spread and frequency selective fading, efficient spectrum usage, simplified sub-channel equalization, and good interference properties.
Although OFDM exhibits these advantages, conventional implementations of OFDM also exhibit several difficulties and practical limitations. One difficulty is the issue of determining and correcting for carrier frequency offset, a major aspect of OFDM synchronization. Ideally, the receive carrier frequency, fcr, should exactly match the transmit carrier frequency, fct. If this condition is not met, however, the mis-match contributes to a non-zero carrier frequency offset, delta fc, in the received OFDM signal. OFDM signals are very susceptible to such carrier frequency offset which causes a loss of orthogonality between the OFDM sub-carriers and results in inter-carrier interference (ICI) and a severe increase in the bit error rate (BER) of the recovered data at the receiver.
Many OFDM standards require the transmission of pilots (known values) embedded in the user data. In conventional OFDM systems, it is common to average the pilots' phase information to improve closed-loop carrier frequency offset tracking in a noisy environment. For example, the average of the pilots' phases may be used to derive a carrier frequency offset estimation which, in turn, may be used to adjust the phase rotations of an equalizer's taps such that the effects of the carrier frequency offset are reduced or removed. One drawback to this technique is that, in the presence of a time-varying channel, the phases of the pilots may vary independently. More specifically, all the pilots' phases share a common phase rotation representative of the carrier frequency offset caused by the mis-match between the transmitter carrier frequency and the receiver carrier frequency, as discussed above. However, in the presence of a time varying channel, each pilot phase may also contain an independent phase rotation caused by the transmission channel varying with time. These independent pilot phase rotations can potentially result in a destructive averaging of the pilots' phases which, in turn, may corrupt the derivation of a carrier frequency offset estimation. A corrupted carrier frequency offset estimation may degrade the performance of any processing unit (e.g., an equalizer) that uses the estimation to compensate for the actual carrier frequency offset. The present invention is directed to the correction of this problem.
It is also possible that the frequency of the sampling clock of the receiver will differ slightly from the frequency of the sampling clock of the transmitter. If there is a frequency difference, the FFT window positioning with respect to the received signal can gradually drift over time. The time domain drift will result in a phase rotation of the received OFDM subcarriers in the frequency domain. The phase rotation may generate errors in the user data recovered by the OFDM receiver. The present invention is also directed to the correction of this problem.